The present invention relates to a brake force booster for automotive vehicles and more particularly relates to a brake force booster that is actuatable by way of an operating rod, by way of a valve piston coupled to an electromagnet.
A brake force booster of this type is disclosed in international patent application WO 94/11226, for example. In this prior art brake force booster, the working chamber can be ventilated independently of the position of the brake pedal or of the operating rod connected to the brake pedal by activation of the electromagnet. A corresponding actuation of the electromagnet permits achieving a so-called xe2x80x98brake assist systemxe2x80x99 which, in the case of an emergency braking, will provide the full boosting force even if the driver does not apply the brake pedal with the pedal force that is required for response of an ABS control system. To this end, the speed of the brake pedal application is sensed e.g. by way of a pedal travel sensor with a subsequent signal differentiation. As soon as the actuating speed of the brake pedal exceeds a predetermined value, braking with full boosting effect is initiated by activation of the electromagnet. A separate release switch is integrated in the conventional brake force booster to deactivate the brake assist system. The release switch will deactivate the brake assist system as soon as the driver releases the brake pedal. However, a concept of this type necessitates relatively great efforts in terms of construction and circuitry due to the pedal travel sensor with a corresponding signal differentiation and due to the additional release switch with the associated trigger mechanism.
An object of the present invention is to provide a brake force booster with an electrical independent actuation which is structurally simplified and easy to actuate electrically, and a method of the booster""s actuation.
According to the present invention, a brake force booster of the initially mentioned type is characterized in that there is provision of a sensor which detects the relative displacement between the valve piston and the control housing, and the electromagnet is activated and controlled in dependence thereon.
The brake force booster of the present invention makes possible a very simple control of the brake assist function because the signals of the sensor can be used both for the activation and the deactivation of the electromagnet. This means that a brake assist function can be triggered, and also terminated, only on the basis of the signals of one single sensor. To initiate the operation of the brake assist system, no pedal travel sensor with a complicated signal differentiation is necessary, which is in contrast to conventional designs. The activation of the electromagnet can be effected when a predetermined first relative displacement between the valve housing and the control housing is exceeded and can be detected in a simple way by a correspondingly designed sensor. The termination of the brake assist function can also take place on the basis of the sensor signals by deactivating, for example, the electromagnet when the amount of displacement falls below a predetermined second relative displacement. Thus, there is no need for a release switch along with its complicated actuation mechanism that is used in conventional electrically assisted brake force boosters. Cost reductions are made possible thereby.
A suitable embodiment of the present invention is characterized in that the actuating current of the electromagnet in the activated condition is variable in response to the relative displacement detected by the sensor. This permits precisely controlling the braking pressure also in the brake assist mode. The driver is thereby enabled to controlledly reduce the braking pressure by correspondingly releasing the brake pedal, without abruptly interrupting the brake assist function. It is this way possible to optimize the withdrawal from the brake assist mode which occurs suddenly in conventional brake force boosters, and to make it largely imperceptible to the driver.
The first relative displacement predetermined for the activation of the electromagnet may be a fixed value selectable in conformity with requirements. However, it may also be variable in dependence on the vehicle deceleration and/or the vehicle speed and/or the braking pressure and/or the time which has passed after the moment of pedal application. This permits optimally adapting the activation of the brake assist system to different driving situations.
The sensor for detecting the relative displacement between the valve piston and the control housing is suitably a non-contact sensor. It may e.g. be a Hall element, an incremental travel pickup, a linear potentiometer, or a like element. In a structurally favorable design, for example, a first sensor element is arranged on a guide member connected to the valve piston, and a second sensor element is arranged on the inside wall of a member which is rigidly connected to the control housing. However, it is also possible to arrange the sensor elements in other locations where a relative displacement between the control housing and the valve piston or other parts of the brake force booster which are moved along with the control housing or the valve piston can be detected.
A particularly simple and expedient electric circuit for activation and deactivation of the electromagnet in dependence on the signals of a sensor which detects the relative displacement between the valve piston and the control sleeve comprises two comparators for actuation of two relays that are connected in series to the electromagnet. One comparator is set to a low value of a relative displacement which corresponds to a disconnecting threshold, and the second comparator is set to a high value thereof which corresponds to a connecting threshold. Only the first comparator is enabled in the ready position of the brake force booster in which the disconnecting threshold is exceeded. When the connecting threshold is reached upon a quick brake application, the second comparator will also become enabled and the magnet for switching on the brake assist system is activated. Additionally, a self-holding function is activated so that the brake assist function will only be discontinued when the disconnecting threshold is reached. A circuit of this type requires only a supply voltage of e.g. 12 volt and can even be integrated in the construction unit of the brake force booster.
Further special characteristics and advantages of the present invention can be taken from the following description of favorable embodiments making reference to the accompanying drawings.